When you say it won't take gas, what do you mean? Does it just stay idling, Stall out, or something else?
Is the check engine light on? Did you check for codes? Did you use the computer that was originally with the engine that you installed? Did everything hook back up exactly to fit the wiring harness? Why did you change engines?

I know that is a lot of questions but cars today aren't like they used to be. It used to be that if the engine ran you could tip the throttle plates open, the vacuum would suck the gas in, and the engine would rev. Now when you tip the throttle plates the computer has to see a lean condition based on many inputs, throttle position, load, etc., and it decides how much fuel to inject to change the RPMs. You could have something as simple as clogged fuel filter, pinched fuel line, blocked air filter, or restricted exhaust. It could also be that a 16 valve engine doesn't use the same computer and/or sensors as the 8 valve.

Get back to us with those answers & any codes that you find & it might help us provide better advice.